U.S. officials suspect that Abu Sufian bin Qumu, who now leads Ansar al-Sharia in Darnah, Libya, took part in the deadly attack which took the life of Ambassador Chris Stevens and three other Americans.

From WaPo:

Witnesses have told American officials that Qumu’s men were in Benghazi before the attack took place on Sept. 11, 2012, according to the officials. It’s unclear if they where there as part of a preplanned attack or out of happenstance. The drive from Darnah to Benghazi is several hours.

The State Department is expected to tie Qumu’s group to the Benghazi attack when it designates three branches of Ansar al-Sharia in Darnah, Tunisia and Benghazi as foreign terrorism organizations in the coming days.

Despite some overlap between Qumu and the "core" Al-Qaeda group founded in Pakistan, the same U.S. officials speaking to The Post said there wasn't evidence that group was directly responsible.

“The situation on Sept. 11th in Benghazi was a complicated one,” a senior administration official told Goldman. “We will never be able to know what motivated everyone involved in this attack, and one of the things the investigation is looking at right now is the level of planning that may have gone into it.”

There is still heated debate over whether the attack was pre-planned or spontaneous, but journalist Matthew Van Dyke, who has been on the ground in Libya, believes an attack of such magnitude could have happened with little planning.

"The people up in [Libya's] green mountains, the extremists, they saw their opportunity to pounce," he told Business Insider in December. "I've experienced how quickly the mobilization can happen firsthand. All it takes is a couple cell phones. All of sudden there's a handful of trucks packed with fighters."